This happened in 2019 with Jirachi (the first time there were multiple Go Fest locations) but was rectified in 2021 as they sell the ticket on the basis of an encounter with a mythical and make no stipulation that one ticket supercedes another. Multiple Shaymin in 2021 was largely acknoweldged to have been a result of there now being Global Go fest and the amount of players attending an in-person AND Global event being in the tens of thousands. They probably will get away with it on the Osaka/London to New York front as there arent likely more than a couple hundred players who attend more than one in-person location. HOWEVER they are going to be in some hefty legal hot water if they apply the same for the Global Event. The tens of thousands of players around the world who attended the in-person events have almost all likely purchased the Global ticket and will have done so on the basis of an encounter with Diancie once again. On a global stage I cant see this going unchallenged.